Required Practical 8: Plant Responses

What equipment is needed for the plant responses practical?

• White mustard seeds

• 3 Petri dishes

• Cotton wool (or cotton pads)

• Ruler (15cm or 30cm)

• Forceps

• Water

• Light source (for full or partial sunlight)

• Labels

What is the method for investigating phototropism?

1. Soak equal bits of cotton wool in equal amounts of water

2. Distribute cotton wool evenly in 3 Petri dishes

3. Place 10 mustard seeds in each dish using forceps

4. Put dishes in a warm place; do not move

5. Once germinated, remove excess seedlings so each dish has same number

6. Measure seedling height in mm

7. Place dishes in different light conditions:

   • Darkness

   • Partial sunlight

   • Full sunlight

8. Measure height for at least 5 consecutive days and record in separate tables

What are the independent, dependent, and control variables in the plant responses practical?

• Independent variable: light condition (darkness, partial sunlight, full sunlight)

• Dependent variable: height of seedlings (mm)

• Control variables: plant species, number of seeds, distribution of seeds, volume of water, temperature, time

What is phototropism?

• Growth of a plant shoot towards or away from light

• Light from one direction causes auxin to accumulate on shaded side

• Cells on shaded side elongate more → stem bends towards light

What tips help during the plant responses practical?

• Keep water volume consistent for all dishes

• Do not move dishes once placed

• Measure height at same time each day

• Use equal number of seedlings per dish for consistency

What are sources of error in the plant responses practical?

• Unequal exposure to sunlight among seedlings in the same dish

• Temperature differences affecting growth

• Uneven water distribution or cotton wool dampness

What is an alternative experiment to investigate gravitropism?

• Set up two Petri dishes with the same number of mustard seeds

• Once germinated, place one on a clinostat and the other on its side in complete darkness

• Record direction of shoot and root growth

• Clinostat rotates shoots to cancel out gravitropism effects

What is gravitropism?

• Auxin accumulates on the lower side of plant

• Shoots: high auxin → more cell elongation → shoot bends away from gravity

• Roots: high auxin → inhibits cell elongation → root bends towards gravity

What happens to seedlings grown in darkness vs light?

• Darkness: seedlings grow taller with smaller, yellow leaves (cannot photosynthesise)

• Partial/full sunlight: seedlings grow shorter, leaves green and able to photosynthesise

• Partial sunlight may cause uneven growth depending on light direction